add cli arg for setting output file

[tschuy]

Super simple arg.

Currently, this will overwrite the file if you present a file that already exists. Personally, I like this as the dev, it makes testing as easy as --output=/tmp/mayday.tar.gz and suddenly I don't have to deal with a billion 1.5MB files cramming my temp space. (if you only care about the log, setting it to /dev/null also works!) However, I don't know if this is ideal for actual usage.

Thoughts? If that's alright, then I think this is pretty much good to go.

[tschuy]

should we also let people include the date and hostname in their custom names with placeholders like {hostname} and {time}?

EDIT: I also set the default hostname, if it errors, to unknownhost, since this doesn't seem like the kind of thing we need to prompt the user about happening.

[tschuy]

Fixes #42

[mischief]

@tschuy if someone wants to use the hostname/time, they can do mayday -o mayday-$(hostname)-$(date -u +%s).tar.gz or something like that.

LGTM

[brianredbeard]

Concur. LGTM.